UV unrawpp keep getting bent ><
you could you use this addon:

https://github.com/Radivarig/UvSquares

it is made for this specific purpose. While the addon is free you can consider supporting the developer if you like and use it: 

https://blendermarket.com/products/uv-squares/

for how to use it:
https://www.youtube.com/watch?v=VYZnGIql2UI